Historical Return Periods of the Strongest Atmospheric Rivers on the U.S. West Coast

Atmospheric rivers (ARs) come in all sizes, and clear communication of strengths of individual storms in observations and forecasts can be a challenge. Modest ARs can be characterized by their ranks within percentiles of the integrated water-vapor transport rates (IVTs) of past ARs, but strong ARs are harder to categorize for many in the public. Strong ARs can be described by analogy to memorable storms in the past, but the number of truly well-remembered storms is generally limited. Alternatively, strong ARs can be categorized more directly in terms of return periods or, equivalently, historical probabilities that at least one AR will exceed a given IVT threshold in any given year. Using a 37-yr chronology of AR landfalls on the US West Coast based on the NASA MERRA (reanalysis) dataset, peak-annual instantaneous and storm-total AR IVTs have been identified, ranked, and used to estimate such return periods along the MERRA 0.5-degree latitude-grid between 32-49oN, categorizing strong ARs with return periods from about 1 yr to almost 50 yr. The strongest (rarest) ARs have historically made landfall between 41-46oN (northern California-Oregon), with maximum IVTs declining to the north and, especially, to the south. The largest (rarest) storm-total IVTs have occurred nearer 38oN (near San Francisco) and 47oN (near outlet of Columbia River), while maximum storm-total IVTs of more common ARs occurred between about 43-45oN (southern Oregon). In autumn, the largest IVTs are occur in the Pacific Northwest, with large IVTs historically tracking southward to about 41-43oN (California-Oregon border) during winter and early spring.